It is known, especially for packaging containers such as tin-plated cans, to provide a decorative pattern upon the external surface and, of course, to protect internal and external surfaces against corrosion by the application of anticorrosion coatings. Both are particularly important when the container, e.g. a can is to be used for the packaging of comestibles or beverages.
The reference to a "decorative" pattern contained herein will be understood to include patterns which are informative, i.e. printed information, patterns which are merely esthetic (e.g. decorations or illustrations) and patterns which are a hybrid of the two, i.e. printed information associated with color fields to enhance certain information. The information which may be encompassed may include the name and address of manufacture, information regarding the source of and nature of the product and consumer information, data or data required by law to be associated with packaging which may reach hands of consumers.
In the past, the application of a decor or pattern upon the outer periphery of the can and the coating of the external surface with a lacquer was carried out in practice by the following sequence of steps:
(a) preparation of the surface of the can and the coating thereof with a lacquer to serve as a ground coat for the decor to be subsequently applied; PA1 (b) drying and/or cross-linking the ground lacquer or primer; PA1 (c) application of the decor or print to the primer on the outer surface of the can; PA1 (d) drying and/or cross-linking or curing of the printed pattern; PA1 (e) spraying the bottom with a lacquer or corrosion protection; PA1 (g) spraying the interior of the can with a lacquer as a corrosion-preventing coating; PA1 (g) drying and/or cross-linking or curing of the anticorrosion lacquers.
When two-part or two-layer lacquer coatings are required, each additional lacquer application step generally requires two distinct operations, for example, a further internal spray lacquering and an additional drying or cross-linking operation.
The very large number of steps required to apply decorative patterns and anticorrosion coatings to metallic articles such as tin cans, requires comparably complex and expensive equipment and a comparatively long process line. Indeed, the earlier technique also requires comparatively large amounts of lacquer since in the application of the lacquer, variations in the coating thickness cannot be avoided and narrow tolerances cannot be maintained so that in many cases considerably greater thickness of the lacquer coating must be employed to ensure effective anticorrosion protection above the minimum thickness layer which is required.
This also applies to the primer-lacquer or ground-lacquer coating to the extent that this coating is not covered by the print or pattern.